test(docs): align boundary-language gate with existing reusable-workflow paths

## Problem

`npm run test:docs-boundary-language` fails on current `main` because the public docs corpus already contains paths matching `/agents\/[a-z0-9._/-]+/i` in `docs/agent-task-reusable-workflow.md` (including `agents/example.agent.json` and `agents/chat`).

## Expected

The docs and boundary assertion should agree: either replace those public path examples with contract-neutral examples or narrow the assertion to the private path language it intends to reject.

## Reproduction

`npm run test:docs-boundary-language`

This was observed while running the #2060 validation matrix and is unrelated to that provider implementation.

Start with docs/agent-task-reusable-workflow.md and run npm run test:docs-boundary-language to reproduce the failure. Compare the public path examples with the boundary assertion, then make the docs and assertion agree through one of the two approaches described. Done means the boundary-language test passes on current main.

Written by the indexing model from the issue text.